If the listing did not explicitly state the adapter was not included then you should have one in the box. Do not spend any extra money. In short the seller has cocked up, but don't give up yet...
Amazon Marketplace are actual shops rather than individuals like e-bay, if you e-mail the seller you brought from they will probably be more than happy to help you. There is a good chance they are unaware of the missing part themselves, if it's an opened box or an unwanted return rather than a product that's actually been used (by Amazon T+C's opened boxes fall under used/refurbished). Give them the benefit of the doubt until they give you a useless reply. Then you can go postal on their ass
Failing that, contact Netgear directly and explain the situation. They may be able to help with a spare part or, if in a really good mood might even send you one free of charge (althouth the fact that it's second hand probably means you'll have to pay for it).
In short, calm down. I know it's a pain in the arse when things go wrong, but this could be a genuine mistake.
Also remember marketplace sellers probably work to 9-5 hours, don't expect a reply instantly. Give it two or three days.
Or if they have a phone number, call them. That'll probably be the quickest way.
